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HTTP content negotiation provides mechanisms for choosing among available HTTP representations, for example different formats, languages or content encodings. One resource can have several representations without requiring that its identifier change for every preference.^[ref]

For a concrete example, stipulate that /color offers both an HTML representation and plain text. A client sends Accept: text/html;q=0.8, text/plain;q=0.5. The weights express a preference for HTML. If this server honors that ranking and both choices are available, it selects HTML and declares Content-Type: text/html. The Accept request field describes preferences; the Content-Type response field identifies the selected media type.^[ref]

The quality weight q lies between zero and one, defaults to one when omitted, and uses zero for an unacceptable choice. These weights are preference values, not probabilities. The example stipulates a server policy; it is not a claim that all servers must select by a universal maximum-weight algorithm.^[ref]

In HTTP proactive negotiation, the server makes its choice using information from the request. A response field Vary: Accept indicates that the request's Accept value might influence selection. This matters when reusing a stored response: the resource identifier alone need not describe every relevant selection input.^[ref]

In HTTP reactive negotiation, the client selects after receiving an initial response containing alternatives, potentially making a further request. This can expose the choices directly but can require an additional exchange. Neither mechanism guarantees that every requested representation exists, nor that the server knows the user's eventual purpose.^[ref]

HTTP proactive negotiation is HTTP content negotiation in which the server selects an HTTP representation using information in the client's request. Explicit preferences can describe acceptable media types, languages or content encodings; the server can also use implicit request characteristics. The specification also calls this server-driven negotiation.^[ref]

Suppose /report offers both JSON and HTML. A client sends Accept: application/json, text/html;q=0.5. The omitted quality weight for JSON defaults to one, so the request expresses a stronger preference for JSON than for HTML. If the server honors this ranking and both forms are available, it returns the JSON form with Content-Type: application/json.^[ref]

This can deliver the preferred representation in the first response. The client does not first need to retrieve a menu of alternatives and then make another choice. In contrast, HTTP reactive negotiation leaves selection to the client after an initial response identifies alternatives. Either approach still requires the application to decide what the available representations mean.

A server selecting on Accept can send Vary: Accept. That field identifies an additional request dimension that might affect the response, beyond the method and target identifier. A stored response chosen for one Accept value is consequently not automatically the appropriate selection for another; caching must account for the relevant variation.^[ref]

The server's information is incomplete. A supported format need not be the best format for the user's intended task, and describing capabilities in every request can increase message size and disclose information about the client. The specification also permits servers to disregard negotiation preferences rather than return 406 when honoring them would leave no acceptable choice. The example's ranking policy is therefore a stated application assumption, not an unconditional protocol guarantee.^[ref]

HTTP reactive negotiation is HTTP content negotiation in which the client selects a representation after receiving an initial response identifying alternatives. The mechanism can be as simple as links to the available forms. The specification also calls this agent-driven negotiation.^[ref]

For an invented example, a client requests /report. The response presents links to /report.html and /report.json, describing the available HTML and JSON forms. A person or client program chooses JSON and requests /report.json. The server then returns that representation. The first response can already contain useful content alongside its alternatives; reactive selection does not require it to be an otherwise empty menu.

The specification discusses alternatives in responses with status 300 or 406, but the defining feature is the client's selection after the first response. The example's suffixes are application conventions: an HTTP representation's actual media type is described by its response metadata, rather than established by a filename alone.^[ref]

An extra request can add delay. In a toy model with an established connection, $80$ milliseconds per complete request/response exchange, negligible processing and transmission time, and no existing copy of the alternatives, fetching the menu and then a chosen representation takes $160$ milliseconds. A correct HTTP proactive negotiation choice in one exchange would take $80$ milliseconds under those same assumptions. These numbers illustrate the additional dependency, not a universal measured performance difference.

The tradeoff can be worthwhile when the client knows requirements that the server cannot infer, or when alternatives can be reused effectively. The server exposes the choices instead of guessing the user's preference. The standard does not prescribe a general automatic selection algorithm for the client, so providing alternatives does not itself establish that every client will make or follow a choice.^[ref]